One technique for simultaneous bi-directional data transmission is separate communication channels. This can be inefficient, though, as available channel capacity in the opposite direction can be unused. Such inefficiency can be costly, particularly where bandwidth is limited (e.g., wireless communication), the transmission distance is long (e.g., long-haul fiber optic communications), or the number of available channels is limited or costly (e.g., high-throughput integrated circuit packages and printed circuit boards (PCBs).
Another technique for simultaneous bi-directional communication is to assign different carrier frequencies for signals traveling in opposite directions. The near-side transmit signal can be modulated by one carrier frequency, and the received signal from the far-side is modulated by another carrier frequency. The difference between the carrier frequencies can be enough so the transmit and receive spectrums do not overlap and can be properly filtered out at each receiver. This method can be inefficient because it splits the effective available channel bandwidth between the two directions, which can be disadvantageous in high-throughput applications. Additionally, the required frequency separation margin, for proper channel isolation and filtering of signals in each band, can lead to wasteful use of the channel bandwidth. For example, operation of a simultaneous bi-directional communication using separate frequency channels in each direction can waste half or more of the total available channel bandwidth.
Another technique for simultaneous bi-directional transmission operates transceivers at opposite ends of a channel medium, concurrently transmitting signals to one another other, through the channel medium, such that the receiver of each transceiver receives a superposition of the signal sent by the opposite end transceiver, and the signal transmitted by its own transmitter. One technique for removing the signal transmitted by its own transmitter is to generate, locally, a replica of that transmitted signal and then subtract the replica from the superposition, then sample the residual. If the replica is exact, the only signal remaining in the residual is the signal from the opposite end transceiver. However, there can be problems with this technique, particularly at higher communication bandwidths. Certain of the problems can arise from fast slew rate transitions of the transceiver's own transmitted signal, as these can introduce noise, especially if proximal in time to a trigger or sampling phase of the receiver's sampler. However, conventional techniques cannot simply shift the sampling phase to move it away from transitions in the locally transmitted signal, because the sampling phase is optimized in relation to the symbol period in the received signal. Merely shifting the sampling phase could result in sub-optimal sampling, which in turn could produce unacceptably high error rates.
There is a need therefore for a practical, stable performance simultaneous bi-directional transceiver system in which opposite end transceivers can each cancel receipt of their own transmissions, while maintaining in the each of the transceivers an optimal sampling of the signal received from the other.